Are you planning to use slugs or buck shot?
Buch shot is best fired through a modified or full choke barrel.
Slugs are usually used in cylinder bore, modified cylinder, or modified choke barrels.
You said your shot gun has open sights and that makes me think it is a cylinder bore, but I could be wrong. My preference for slugs in a smooth bore are Brenneke KO slugs (12 ga. 2 3/4" 1600fps) and Fiocchi 12 ga 1 oz slug with attached wad. Both slugs have the attached wad that stablizes the slug. You may be surprised how accurate a smooth bore can be with these slugs. You can order them both from
www.ballisticproducts.com.
Good luck
Tarheel